In recent years, human therapeutics have expanded past traditional small molecule drugs and into the realm of biopharmaceuticals. The discovery of novel proteins and peptides has led to the development of numerous protein and polypeptide biopharmaceuticals. Unfortunately, proteins and polypeptides, when utilized as therapeutics, often exhibit properties that make them extremely difficult to formulate or administer, such as short circulating half lives, immunogenicity, proteolytic degradation, and low solubility.
There is significant interest in using naturally occurring biopolymers as drug delivery and/or conjugating agents, as they are generally safe and because they are biodegradable. Both of these factors are very significant in selecting a polymer for drug delivery, as polymer safety and clearance rank among the most important issues in selecting a delivery polymer. Solubility in serum and in aqueous media is often very important for drug manufacture and drug delivery. By far the most successful water soluble polymer in use for drug delivery is polyethylene glycol (PEG) (see e.g. Harris, J. M. (1985), “Laboratory Synthesis of Polyethylene Glycol Derivatives”, JMS-Rev. Macromol. Chem. Phys. C25: 325-373; Harris, J. M. and Chess, R. B. (2003), “Effect of PEGylation on Pharmaceuticals”, Nature Reviews Drug Discovery 2: 214-221; Roberts, M. J., Bentley, M. D., and Harris, J. M. (2002), “Chemistry for peptide and protein PEGylation”, Adv. Drug Del. Rev. 54: 459-476; Pasut, G., Guiotto, A., and Veronese, F. M. (2004), “Protein, peptide and non-peptide drug PEGylation for therapeutic application”, Expert Opin. Ther. Patents 14:1-36; Filpula, D. and Zhao, H. (2008), “Releasable PEGylation of proteins with customized linkers”, Adv. Drug Del. Rev. 60: 29-49; Zhao, X and Harris, J. M. (1997), “Novel degradable poly(ethylene glycol) esters for drug delivery”, In: Harris J. M. and Zalipsky S. (eds): Poly(ethylene glycol) chemistry and biological applications, American Chemical Society, Washington, D.C., 458-472.)
Attempts have also been made to use carbohydrates for conjugation with and delivery of drugs. The most prominent examples are hydroxyethyl starch (HES) and polysialic acid (PSA). HES, a derivative of naturally occurring starch (amylopectin & amylose), has been described as the polymer of choice as a polyfunctional carrier for oligopeptide-polymer conjugates. It is non-toxic and nonimmunogenic and is degraded by α-amylase in the body.
HES is recommended as a polyfunctional carrier, since it has a large number of functional groups (primarily hydroxyl groups), making monofunctionalization virtually impossible. Thus, activation of HES for attachment of drug moieties provides a polyfunctional polymer with a diversity of sites; i.e. some polymer molecules have more functional sites than other polymer molecules, and the overall result is a polydisperse distribution of reactive sites. This is often acceptable for smaller drug molecules, where several drug molecules per polymer strand may be acceptable, as long as biodistribution is not affected by the polydisperse character of the conjugate. However, it is undesirable for larger molecules such as proteins, where a single protein molecule per polymer moiety is highly desired. In fact, in many cases, e.g. with very large proteins like Factor VIII, multiple polymer molecules may be desired to protect the protein. While PEGs are very suitable for such applications, because they can readily be engineered to provide only one active group per polymer molecule, it is very difficult to selectively activate only one functional group of a carbohydrate polymer.
Polysialic acid (PSA) and hyaluronic acid (HA) are acid carbohydrates that have also been promoted for drug delivery. However, the very high biodegradability of HA is problematic, as it is generally inadequately stable in vivo to be a good delivery agent. HA also suffers from its strong targeting properties which consistently steer it toward certain biotargets. PSA has enjoyed more recent attention. Like HES, however, PSA is only reasonably useful when polyfunctionality is acceptable. Thus, to this point, polysaccharides have not presented a significant commercial threat to the use of PEGs (PEGylation) for modification of the pharmacological properties of drugs; i.e. relatively little product development activity has occurred, and there are no launched products.
Another issue that must be dealt with in employing carbohydrates for drug delivery is control of molecular weight. It is possible, for example, to manufacture PEG with fairly precise control of molecular weight and polydispersity. Thus, it is common to see commercially available PEGs having moderate molecular weights (i.e. 20-40 kD) with a polydispersity of 1.10 or less. Since carbohydrates are biologically derived, often with variable molecular weights, obtaining a specific molecular weight for drug delivery is difficult. With respect to molecular weight distribution, commercially available dextrans often have polydispersity values of 2.0 or greater, or around 1.25-1.35 for purified materials.
Thus, there remains a need to overcome these barriers of polyfunctionality and polydispersity in order to take advantage of the desirable properties of these naturally occurring polymers.